A Long Term Evolution (LTE) system includes an evolved packet core (EPC), an evolved NodeB (eNodeB), and user equipment (UE). The EPC is a core network part, and includes a mobility management entity (MME) that is responsible for signaling processing and a serving gateway (SGW) that is responsible for data processing. The eNodeB is connected to the EPC by using an S1 interface, eNodeBs are connected by using an X2 interface, and the eNodeB is connected to the user equipment by using a Uu interface.
An evolved universal terrestrial radio access network (E-UTRAN) includes eNodeBs and is responsible for implementing a radio-related function. An E-UTRAN protocol framework includes a user-plane protocol and a control plane protocol. A user-plane protocol stack includes a Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and a Media Access Control (MAC).
Referring to FIG. 1, in the prior art, a data packet may be transmitted from a PDCP entity to an RLC entity. The RLC entity may include a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The RLC entity shown in FIG. 1 is an acknowledged mode RLC entity.
For a carrier aggregation (CA) scenario of an ideal backhaul heterogeneous network (HetNet), in the acknowledged mode of the RLC, data packets may be offloaded to a macro eNodeB to which a primary cell (Pcell) belongs and a micro eNodeB to which a secondary cell (Scell) belongs, so as to separately send the data packets to user equipment (UE) by using the primary cell and the secondary cell, and improve data sending efficiency.
After receiving the data packets, the user equipment sends, to the macro eNodeB, feedback information for the data packets transmitted by the macro eNodeB and the micro eNodeB. The feedback information may generally include two types: an acknowledgement (ACK)/a negative acknowledgement (NACK). For example, if feedback information that is received by the macro eNodeB for a data packet transmitted by the micro eNodeB is a NACK, the macro eNodeB generally sends the NACK to the micro eNodeB, and the micro eNodeB generally processes the data packet in a manner of performing a hybrid automatic repeat request (HARQ) by using the secondary cell.
However, when the micro eNodeB performs the HARQ on the data packet, if all HARQ processes have been occupied, no data packet can be retransmitted until a HARQ process is idle. This increases a retransmission delay, causes a relatively low throughput of the user equipment, and reduces efficiency of the HARQ.